By the spring of 2010, the veteran teacher had spent his entire 16-year career at Charles Seipelt Elementary School. Like other Seipelt teachers, Mueller regularly wrote study guides to prepare his classes for upcoming state tests.

On test day last April, several fifth-grade students immediately recognized some of the questions on their math tests. They were the same as the questions on the study guide Mueller had given them the day before. Some numbers on the actual tests were identical to those in the study guide and the questions were in the same order, the kids told other teachers.

Ultimately, investigators concluded that Mueller had looked at questions for both fifth-grade math and science tests in advance - a violation of testing rules - and then copied them, sometimes word for word, into a school computer. That's how he developed his study guides.

The 50-year-old teacher resigned. He signed a state document admitting that, by looking at the tests in advance to prepare the study guides, he had "engaged in conduct unbecoming a licensed educator." His teaching license was suspended for three months.

At Seipelt, as in hundreds of schools nationwide where teachers have been found cheating on standardized tests, young students tipped off officials that something was amiss. Yet if anyone had taken a close look at the past few years' test scores, they would have noticed Seipelt.

In several grades, standardized test scores at Seipelt fluctuated from year to year, sometimes rising sharply, then falling just as fast, according to data USA Today obtained from the Ohio Department of Education.

In 2005, for example, the school's third-graders tested in the 67th percentile statewide in math. As fourth-graders a year later, when Mueller was one of their teachers but didn't teach math, their scores jumped to the 97th percentile, among the best in the state. As fifth-graders in 2007, the scores plunged to the 49th percentile. Then, in 2008, when they were in sixth grade, their scores climbed again to the 90th percentile.

Two of Seipelt Elementary's score fluctuations - the fifth-graders' 49-percentile decline in 2007 and the 41-percentile climb in sixth grade a year later - registered more than three standard deviations. School officials attributed the fifth-grade decline to inexperienced teachers and the sixth-grade jump to a teacher with 25 years on the job.

Milford district officials said they saw no reason to suspect that any of their teachers cheated before 2010. Robert Farrell, superintendent of the 6,600-student district, said he considered Mueller's transgression a one-time event.

The fifth-graders who used Mueller's study guides had to retake last year's math and science tests, at a cost of $3,300 to the district. They passed the math test at about the same rates as earlier fifth-graders; 91 percent were proficient or better. About 81 percent passed the science test, down 16 percentage points from the year before.

Mueller's teaching license was suspended for three months. He did not respond to requests for comment.